Considerable interest has recently developed in the production of phosphazene compounds (also known as phosphonitrilate ester compounds) as fire retardants for cellulosic materials, as plastics with exceptional solvent and chemical resistance, for treatment of polyesters and as elastomers. Of particular interest is production of alkoxy and aryloxyphosphazenes, especially hexapropoxyphosphazene (HPP). Several methods of preparation of alkoxy and aryloxyphosphazenes are known. In general, the reaction schemes employed include the reaction of a phosphonitrilic chloride with a hydroxy compound or metal alcoholate to produce the desired phosphazene compound. In Stokes, American Chemical Journal, Vol. 19, p. 782 (1897) and Schenck et al, Berichte, Vol. 57B, p. 1343 (1924), the formation of phosphonitrilic chlorides by the reaction of phosphorus pentachloride with ammonium chloride is taught. More recently, U.S. Pat. No. 3,656,916 to Schiedermier et al teaches the reaction of ammonia with phosphorus pentachloride to produce phosphonitrilic chlorides. The compounds react with alcohol or the sodium alcoholate prepared by known methods, such as reacting excess alcohol with molten sodium in a suitable solvent, to prepare the desired phosphazene compound.
Further illustrating processes and the types of materials which are suitable for recycle to the manufacture of phosphazene compound are U.S. Pat. No. 3,795,526 to Bergeron in which a condensed cyclic alkoxy or aryloxy phosphazene is produced starting with a phosphonitrilic chloride and a hydroxy compound, such as monohydric aliphatic and aromatic alcohols or the alkali metal derivatives thereof, and an alkali metal hydroxide which aids in the condensation. Solvents employed include aromatic hydrocarbons, such as benzene, toluene, xylene and chlorobenzene; aliphatic hydrocarbons such as heptane; and commercially available mixtures such as kerosene, No. 9 oil, etc. Also unreacted hydroxy compound such as excess propanol must be removed from the product or intermediates therefor. In U.S. Pat. No. 3,836,608 a partially esterified phosphonitrilic chloride, prepared from the reaction of phosphonitrilic chloride and an alcohol or alkoxide, is heated and self-condenses to form a condensed polymeric phosphazene with P-O-P linkages. Solvents used include octane or chlorobenzene. Impurities are removed, such as by filtration and washing. Solvent is removed by heating in vacuo (e.g. 85.degree., 16 torr) to concentrate the partial ester. However, no mention is made of recovering the solvent or unreacted alcohol. On condensation, an alkyl halide is evolved. There is likewise no mention of recovery of the alkyl halide. In U.S. Pat. No. 3,840,621 an esterified phosphazene is produced by reacting a chlorophosphazene with ethylene glycol or other low molecular weight glycol, and if desired, further esterifying with alcohol, alkoxide, or alkylene oxide. Aromatic and paraffinic solvents are employed and amines, such as triethylamine, are used to complex with the HCl liberated on esterification. Removal of the solvent, amine and unreacted alcohol is required but no method of recovery is suggested. In U.S. Pat. No. 3,844,983, a poly(fluoroalkoxyphosphazene) copolymer is produced by reacting a poly(dichlorophosphazene), an alkali metal salt of an alcohol or a mixture of different alcohol salts, and a third alkali metal salt of an alcohol containing an amine function. Solvents employed include benzene, toluene, xylene, chlorobenzene, 1,2-dichlorobenzene, tetrahydrofuran or mixtures thereof. Washing with water and separation of organic and aqueous phases are mentioned for separation of the poly(fluoroalkoxyphosphazene) prior to curing or crosslinking of the polymer, but no mention is made of the recovery of solvent or excess alcohol for recycle. More recently in application Ser. No. 529,760, filed December 5, 1974, now U.S. Pat. No. 3,974,242 issued Aug. 10, 1976 there is taught a process for reacting a phosphonitrilic halide and an aromatic or aliphatic alcohol in the presence of an acid acceptor, particularly a tertiary amine, whereby a condensed phosphonitrilate ester is produced. Excess alcohol, tertiary amine and process solvent such as chlorobenzene must be recovered.
In each of these processes, the use of solvents plays an important role in the process. The earlier work of Stokes and Schenck et al, supra, used symmetrical tetrachloroethane as a solvent for the preparation of the phosphonitrilic chlorides. However, this material is expensive and very toxic. More recently, emphasis for this reaction has been placed on the use of monochlorobenzene. In addition, the reaction for producing the phosphazene has been carried out in the presence of tertiary amines as an acid acceptor because the reaction liberates large quantities of hydrogen chloride. In more recent studies, the use of toluene has been shown to provide acceptable properties in the alkoxyphosphazene reaction. In many cases, however, the result is a mixed solvent system, unless extensive steps are taken to remove the solvent prior to reaction further with the alcohol or alcoholate.
The use of the mixed solvents, the presence of unreacted alcohol and amine complexes poses practical problems in the production of alkoxy and aryloxyphosphazenes. For commercial application a process must not only provide the desired end-product in good quality and good yield, it must also be considered from an economical standpoint in the recovery and reuse of the auxiliary materials such as solvent and unreacted alcohol. The nature of the mixed solvent system provides difficulty in separation especially when it is considered that the solvents must be separated from an aqueous wash system used in purifying the desired phosphazene compound. Previously, it was suggested that extensive liquid-liquid extraction systems requiring large capital expenditures and high processing cost could be used to separate and recover the solvents for recycle to the manufacturing operation. One such system involving the separation and recovery of monochlorobenzene and pyridine is taught in Netherlands Patent Publication 71/06772.
Now a relatively simple and low cost process for recovering solvents, hydroxy compound and tertiary amine acid acceptor suitable for recycle to the manufacture of alkoxy and aryloxyphosphazenes has been found. The present invention allows good solvent separations, provides for low losses of useful materials and, in addition, avoids the use of an elaborate extraction system for separation of organic materials from the aqueous wash system.
It is an object of this invention to provide a process for the recovery of solvent, hydroxy compound and tertiary amine acid acceptor in the manufacture of alkoxy and aryloxyphosphazenes. A further object is to provide a more economical process for the manufacture of alkoxy and aryloxyphosphazenes by recovery and recycle of auxiliary materials. In particular, an object is to provide a process for the manufacture of hexapropoxyphosphazene which does not require an elaborate extraction system to recover valuable organic materials from the aqueous wash stream. These and other objects will become apparent from the following description of the invention.